predominant species

[Annabella_Amato_1I]

one of the learning outcomes for the acid base equilibria is "use pKa values to determine the predominant species of a weak acid/weak base in solution". I'm not sure what it is meant by "predominant species" and how this would be determined by the pKa value?

[Marc Lubman 3B]

The predominant species refers to which side of an acid/base equilibrium equation is favored in a particular solution, i.e. whether a solution will have more [HA] or [A-] (which can also be referred to as a neutral and charged species, respectively). The pKa value is indicative of whether the reactant or product concentration is higher at equilibrium, and can thus be used to determine what the predominant species is. For example, if your pKa is lower than your pH then the predominant species is [A-], and if your pKa is higher than your pH then the predominant species is [HA].